One known thermal type flow measuring device is disclosed in, e.g., JP,A 60-142268. In this disclosed thermal type flow measuring device, a heater is formed in a bridge section formed by etching a silicon wafer, and temperature sensors are formed respectively upstream and downstream of the heater.
Further, a reference-purpose air temperature sensor is formed in a frame area other than the bridge section, and the heater temperature is feedback controlled to be held at a constant value higher than the air temperature on the basis of the reference-purpose air temperature sensor. Then, the flow rate of air is detected based on a temperature difference between the temperature sensors disposed upstream and downstream of the heater.
With the technique disclosed in the above-cited JP,A 60-142268, however, because a slit for thermal insulation is formed in the bridge section and the strength of the bridge section is relatively poor, there is a possibility that the bridge section may be easily damaged if dust in air strikes against the bridge section. Also, the technique disclosed in the above-cited publication is not suitable for use in control of an automobile engine, which requires high reliability under severe environments, for the reason that a variation in characteristics may occur upon dust or the like clogging up the slit formed for thermal insulation.
Another problem has been experienced in that because a resistor is heated to serve as the heater, the resistance value and the resistance temperature coefficient of the heater change with time and characteristics of flow rate also vary with time.
On the other hand, a thermal type flow measuring device disclosed in JP,A 2-120621 represents an example in which a variation of sensor characteristics resulting from time-dependent changes of the heater resistance is compensated by adding an electric circuit.
More specifically, the technique disclosed in the above-cited JP,A 2-120621 employs a circuit arrangement for computing a rise of the heater temperature from outputs of sensors upstream and downstream of a heater, computing a difference between the temperature detected by the upstream sensor and the temperature detected by the downstream sensor, and then producing an output resulting from dividing the temperature difference by the temperature rise.
With such a circuit arrangement, the time-dependent changes of the heater resistance can be compensated by using the upstream and downstream temperature sensors.
Additionally, other known techniques related to the thermal type flow measuring device are disclosed in, e.g., U.S. Pat. No. 5,369,994, JP,A 6-160142 and JP,A 2000-193505.